Curable silicone coatings for release of pressure-sensitive adhesives (PSA) are known. These coatings may be cured via tin-catalyzed condensation reactions, platinum-catalyzed addition reactions, or radiation-initiated crosslinking reactions.
Linear polydimethyl siloxanes form what are referred to as premium or low release coatings. When crosslinked, these materials form adhesive surfaces from which a PSA-laminated facestock may be detached with very little force required. Such premium release characteristic is desirable for many applications.
There are, however, significant applications for silicone release coatings which do not provide easy release from PSA's. Such coatings which provide a tight or controlled release characteristic are desirable, for example, in industrial labeling operations which run at high speed. A high release silicone agent is desirable in order to prevent so-called premature dispensing of the label from the release liner. A controlled release characteristic is useful for two-sided release laminate applications wherein one side of the laminate is coated with a premium release silicone and the other side is coated with a tight release silicone, thus providing a differential release structure.
Ideally, silicone release systems that have a range of release characteristics are desirable. Such silicone coating systems would enable an end-user to selectively adjust the formulation to give the desired level of release depending on the specific application. A tight release polymer capable of giving a range of release levels in a blend with premium release polymer is referred to as a controlled release additive (CRA).
Despite the intensity of effort in this area, commercial products designed to be controlled release (tight release) additives (CRAs) for ultraviolet radiation curable epoxysilicone release systems have not been generally successful because they often do not cure as readily as the premium release epoxysilicones; they do not provide large release differential vs. the premium release materials except at very high concentrations in coating baths (&gt;70%); and cured films containing these CRAs often lose their high release characteristics as they age after cure. Promising experimental UV-curable CRAs have also not been as efficient as desired, and in many cases have caused undesirable `zippy` (jerky, uneven) type release which is unacceptable in the market.
Exemplary UV-CRA products provide tight release (relative to the premium release epoxysilicone polymers taught in U.S. Pat. No. 4,279,717 and other patents) by incorporation of highly polar hydroxybenzoate ester groups (U.S. Pat. No. 4,977,198) or phenol groups (U.S. Pat. No. 4,952,657) along with reactive epoxy groups into the polydimethylsiloxane polymers. In each case, products so designed are not successful for reasons noted above.
UV-CRAs incorporating reactive, epoxy-functional `Q` type silicone resins plus linear (nonresinous) epoxysilicone polymers analogous to traditional thermally cured silicone CRAs (solvent-borne and solventless) which make use of silicone resins as taught in U.S. Pat. Nos. 4,123,604, 3,527,659 and 5,158,911 and in U.S. patent application Ser. No. 07/923,112, filed Jul. 30, 1992, in the name of Eckberg and Griswold. In U.S. Pat. No. 5,158,911, the epoxy-MQ resins taught therein are high viscosity (or solid) materials incompatible with low viscosity epoxysilicone carriers without solvent vehicle. Eckberg and Griswold, which teaches `bound`M' TQ resin/epoxysilicone fluid compositions, is limited in effectiveness because solventless silicone coating constrains coating bath viscosities to be&lt;1000 cstk, which in turn limits the actual resin content of the CRA&lt;35 wt %. High release of these materials is a function of actual epoxysilicone resin concentration. Thus, the efficiency of the compositions is limited.